The present invention relates to a catalyst for purifying an exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons contained in an exhaust gas discharged, for example, from internal combustion engines of automobiles and the like, and in particular, to a catalyst for removing nitrogen oxides contained in an oxygen-rich exhaust gas.
The term "oxygen-rich exhaust gas" used in the present invention is intended to mean an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing carbon monoxide and hydrocarbons and hydrogen contained in the exhaust gas.
2. Description of the Related Art
Nitrogen oxides, carbon monoxide and hydrocarbons, which are toxic substances contained in an exhaust gas discharged from internal combustion engines, are removed, for example, through the use of a three-way catalyst comprising Pt, Rh, Pd, etc., supported on a carrier material. In the case of an exhaust gas discharged from diesel engines, however, no effective catalyst exists for removing nitrogen oxides because the exhaust gas contains a large amount of oxygen, and thus a purification of the exhaust gas by a catalyst has not been realized.
In recent gasoline engines, a lean burn combustion is used for lowering the fuel consumption and reducing the amount of exhausted carbon dioxide gas, but an exhaust gas from this lean burn gasoline engine comprises an atmosphere containing an excessive amount of oxygen, and therefore, it is impossible to use the above-mentioned conventional three-way catalyst, and thus a method of removing toxic components from the exhaust gas has not been put to practical use.
Examples of the method of removing particularly nitrogen oxides in an exhaust gas containing an excessive amount of oxygen include that wherein a reducing agent such as ammonia is added, and that wherein the nitrogen oxides are absorbed in an alkali to remove same. These methods are not effective for automobiles, which are a moving nitrogen oxides source, and thus the application thereof is limited.
Recently it has been reported that a zeolite catalyst subjected to an ion exchange with a transition metal can remove nitrogen oxides in an exhaust gas containing an excessive amount of oxygen, without the addition of a special reducing agent such as ammonia. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 63-283727 and 1-130735 propose a catalyst able to selectively reduce nitrogen oxides even in an exhaust gas containing an excessive amount of oxygen and minor amounts of reducing agents such as unburnt carbon monoxide and hydrocarbons.
The activity of the above-mentioned catalysts proposed in the art, however, is remarkably deteriorated when used at a high temperature for a long time, and thus it is necessary to improve the durability and catalytic performance thereof.
Accordingly, to solve the above-described problems, a catalyst for purifying an exhaust gas comprising a zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of at least 15, and incorporated therein cobalt and an alkaline earth metal, has been proposed (see Japanese Patent Application No. 1-337249).
Although the exhaust gas purification catalyst proposed in Japanese Patent Application No. 1-337249 has an improved durability, the temperature region in which the nitrogen oxides can be removed is relatively narrow. Therefore, a higher capability of removing nitrogen oxides in a broader temperature region, particularly at a low temperature, is required from a catalyst for purifying an exhaust gas discharged, in particular, from automobiles.